Present dietetic needs, predilections, and perceptions have led to the increased use of artificial sweeteners as a replacement for the "natural" sugars, including sucrose and fructose. Such artificial sweeteners are highly imperfect, including being under continual review for their long term physiological affects, yet their demand has grown unabated. Accompanying their growth as a commercial area with substantial economic impact has been a renewed emphasis on discovering and supplying new artificial sweeteners.
The ideal artifical sweetener would be noncaloric, noncariogenic, without detrimental physiological effects, and usable by diabetics. All these requirements would be met if a sweetener were not metabolized by humans and by flora which are found in the mouth and intestinal tract, and if the sweetener were either not absorbed by humans, or absorbed without effect on any internal organ. That is, the ideal sweetener should be excreted in the same form as when ingested. Another desirable feature is that it have bulk properties similar to sucrose so that it can be substituted for table sugar in many formulations. Recently, and perhaps belatedly, attention has turned toward the L-sugars as desirable artificial sweeteners. It has been known since at least 1946 that L-fructose is sweet (M. L. Wolfrom and A. Thompson, J. Am. Chem. Soc., 68, 791, 793 (1946)), and since at least 1890 that L-fructose is nonfermentable (E. Fischer, Ber. Deutsch. Chem. Ges., 23, 370, 389 (1890)), hence not metabolized by microorganisms generally metabolizing D-sugars. A reasonable, although not necessarily correct, inference is that it also is not metabolized by humans. Assuming that L-fructose is a sweet nonmetabolite it becomes obvious to use it as a noncaloric sweetener in many formulations. More recently Shallenberger and coworkers have demonstrated that many L-sugars have a sweetness comparable to their L-enantiomorphs. Nature, 221, 555 (1969). Cf. R. S. Shallenberger, "The Theory of Sweetness," in Sweeteners and Sweetness, pp 42-50, Edited by G. G. Birch and coworkers; R. S. Shallenberger and T. E. Acree in "The Handbook of Sensory Physiology," Vol. 4, pp 241-5, Edited by L. M. Beider (Springer Verlag, 1971).
Exploitation of the favorable properties of L-sugars is hindered by their relative unavailability. L-Fructose, for example, is not found to any significant extent in nature. This unavailability has spurred recent efforts in developing commercially feasible methods for preparing L-sugars in amounts necessary for their use as a staple of commerce. U.S. Pat. Nos. 4,371,616 and 4,421,568 describe a method of producing L-sugars, including L-idose and L-glucose, from the readily available D-glucose. Although the preparation of a number of L-sugars is described in U.S. Pat. No. 4,262,032 the focus seems to be on typical laboratory methods wholly unsuited or economical industrial production, in contrast to the process herein. U.S. Pat. No. 4,440,855 presents a flow scheme for the preparation of a mixture of L-glucose and L-mannose. The subject matter of U.S. Pat. No. 4,207,413 is L-sucrose, the enantiomer of ordinary table sugar, which can be hydrolyzed to afford L-fructose and L-glucose.
Many of the synthetic routes to L-sugars can be based on homologation in which a 1-carbon chain extension is effected by addition of the elements of HCN to an aldose. Conversion of the resulting cyanohydrin to an aldehyde group affords the next higher aldose, usually as an epimeric pair. One method of converting the cyanohydrin to an aldehyde is by catalytic hydrogenation with concomitant hydrolysis of the imine, the reduction product. The catalyst used must be active in effecting the reduction of a nitrile group to an imine, but must show little tendency to reduce either the imine initially formed or the aldehyde resulting from imine hydrolysis. Zerovalent palladium has been found to fill such requirements and often is used in cyanohydrin reduction. Supported palladium usually is preferred to colloidal palladium because of the ease of separation and recovery of the noble metal, as well as for increased catalyst activity. Because reduction of the cyanohydrin is performed under rather acidic conditions the support must be physically and chemically stable in acid solutions, a requirement which precludes the use of, for example, gamma-alumina, an otherwise popular support in catalytic reactions. The required acid resistance has necessitated the use of infrequently used and uncommon supports such as barium sulfate.
From the foregoing it is clear that hydrogenation of cyanohydrins with concomitant hydrolysis of the formed imine to an aldehyde is an uncommon process with uncommon catalyst requirements. To further complicate matters we have found that catalysts useful in the foregoing process are readily poisoned, being rendered effectively inactive after but one batch reduction, i.e., the catalysts cannot be reused. Of course this also implies that a continuous process employing such catalysts also is not feasible. Therefore it became mandatory to search for procedures which would regenerate catalyst activity.
The subject of this application is the regeneration of supported palladium catalysts for the reduction of cyanohydrins with concomitant hydrolysis to afford aldehydes as the final reaction product. More particularly it is directed toward catalyst regeneration when the catalyst is used in the reduction of cyanohydrins produced in homologation of sugars and when the product is an aldose.